The present invention relates to a laminate with superior gas barrier properties, a production method therefor, and a paper container employing this laminate. More specifically, the present invention relates to a laminate with gas barrier properties that can be produced without using an organic solvent, this laminate being excellent for long-term storage, non-refrigerated transport, and aseptic packaging applications due to its superior capacity to act as an oxygen barrier. The present invention further relates to a production method for this laminate, and to a paper container employing this laminate.
This application is based on patent applications filed in Japan (Japanese Patent Application Nos. Hei 10-210080 and Hei 10-210081), the contents of which are incorporated herein by reference.
In general, a laminate having a three-layer structure consisting of polyethylene/paper board/polyethylene is employed for the laminate used in paper containers and the like for packaging liquids (polyethylene will be abbreviated as xe2x80x9cPExe2x80x9d hereinafter). (Note that xe2x80x9cPE/paper board/PExe2x80x9d as employed in this specification indicates that PE, paper board and PE have been laminated together in this order). A laminate of this design is formed into a box-shaped, sealed container by making use of the high rigidity of paper board and the heat sealing properties of the PE which is disposed to the outer and inner surfaces of the paper board. Containers of this design are widely used as 200 ml to 2 liter milk cartons, or as containers for refreshments or alcoholic beverages. As compared to a metal or glass container, however, containers consisting of this type of laminate using paper board and PE permit more oxygen to permeate through the container. Thus, in cases where the quality of the contents within the container could be affected by oxidation, these containers are disadvantageous in that they require chilled transport and only permit storage for a short period of time.
When a container is to be filled with sterile contents or to be transported at room temperature, a packaging material which allows minimal deterioration in contents must be employed. In this case, a paper container consisting of a laminate that has gas barrier properties is required. In addition, depending on the material held, a container may be required to retain the odor of the contents. From this perspective as well, a paper container that has gas barrier properties is needed.
Paper thinner than the paper board used for packaging liquids is employed in the case of paper containers for holding dry food products, medicines or the like. Namely, a two-layer laminated paper consisting of paper and PE layers is employed. Characteristically, a container employing this type of laminated paper is easily torn by hand, provided that the paper has some stiffness and is thin. As in the case of a paper container for packaging liquids, this type of paper container may also be required to have a gas barrier function according to its application.
Laminating aluminum foil (xe2x80x9cAl foilxe2x80x9d hereinafter) is a frequently employed approach for providing gas barrier properties to a paper container. In this case, when taking into consideration adhesiveness between layers and the heat sealing ability of the inner layer of the container, a PE/paper/PE/Al foil/ethylene-methacrylic acid copolymer (xe2x80x9cEMAAxe2x80x9d hereinafter)/PE structure, or a PE/paper/PE/Al foil/PE structure is used.
A container for holding an alcoholic beverage, for example, must have gas barrier properties, a heat-sealing capability on either surface, and have sufficient impact-resistance to prevent damage to the container if dropped. In this case, a PE/paper/PE/Al foil/PET/PE structure may be employed in which the Al foil is used as the gas barrier layer and polyethylene terephthalate (xe2x80x9cPETxe2x80x9d hereinafter) is used as-the impact-resistant resin layer.
A dry laminating method, extrusion laminating method, or the like may be suitably selected as the method employed to produce a laminate having Al foil in this way. Alternatively, depending on the structure of the laminate, both methods may be employed together.
In a dry laminating method, an adhesive agent such as isocyanate is dissolved in organic solvent, and the liquid material resulting is coated to one side of a base material. The solvent is evaporated in a dryer and respective base materials are adhered together using a nip roller. This method requires using a large amount of expensive adhesive agent, and thus increases production costs. In addition, the use of organic solvents has a negative impact on the work environment and results in an unpleasant odor remaining in the final product.
In contrast, the extrusion laminating method does not require an adhesive agent when adhering together the PE and the paper. However, when laminating together PE and a base material consisting of a film, such as PET film, or when laminating together PE and a base material consisting of a metal foil such as Al foil, the usual method is one in which an isocyanate or similar type adhesive agent referred to as an anchor coat agent (xe2x80x9cACxe2x80x9d hereinafter) is coated in advance onto the base material in order to increase adhesion. A resin is then melt extruded onto this coated layer. However, this method not only has higher production costs as was the case of the dry laminating method above, but there is also a deterioration in the work environment due to the use of organic solvents. This method is further problematic in that an odor remains in the final product.
In order to resolve these problems, methods have been proposed which do not employ an AC agent derived from organic solvents, or which employ an aqueous AC agent derived from polyethyleneimine.
However, aqueous AC agents have the disadvantage of inferior water resistance due to the adhesive agent""s intrinsic aqueous solubility.
As approaches which do not employ an AC agent, methods have been proposed in which a polar group, such as a carboxyl group or acid anhydride group, is introduced into polyolefin. Examples of this approach include the methods disclosed in Japanese Patent Application, First Publication No. (A) Sho 57-157724 (WPI Acc No. 82-95762E); Japanese Patent Application First Publication No. (A) Sho 59-75915 (WPI Acc No. 84-143564); Japanese Patent Application First Publication No. (A) Hei 8-252890 (WPI Acc No. 96-492986); Japanese Patent Application First Publication No. (A) Hei 8-258225 (WPI Acc No. 96-501588); Japanese Patent Application First Publication No. (A) Hei 8-258234 (WPI Acc No. 96-501597); Japanese Patent Application First Publication No. (A) Hei 9-66587 (WPI Acc No. 97-221082); Japanese Patent Application First Publication No. (A) Hei9-76419 (WPI Acc No. 97-240491); Japanese Patent Application First Publication No. (A) Hei 10-264342 (WPI Acc No. 98-588752), and the like. While there is an increase in the adhesive strength of the laminate obtained using these method as compared to a laminate formed using polyolefin alone, this adhesive strength is not adequate when compared to the case where an AC agent is used.
In addition, productivity is poor with methods employing dry laminate adhesive agents or extrusion laminate AC agents, making these approaches problematic as well.
For example, when producing the laminate employed in the aforementioned alcoholic beverage container, dry laminating is performed one time in order to laminate the Al foil and PET, tandem extrusion laminating and single extrusion laminating are performed one time in order to laminate the PE, and a urethane AC agent is required to laminate the PET and PE.
In other words, in order to produce a laminate of a structure consisting of Al foil and PET, a dry laminate adhesive agent and an extrusion laminate AC agent are necessary. As a result, production costs increase, a considerable amount of organic solvent is used, and there are numerous steps involved to make the laminate. For example, in the aforementioned alcoholic beverage container, dry laminating is performed one time, tandem extrusion laminating is carried out one time, and single extrusion laminating is carried out one time. In other words, laminating is performed three times. Further, since time is required to cure (aging time) the adhesive agent, productivity is poor. With respect to a laminate and production method therefore which does not employ an anchor coat agent, the present inventors have already proposed the methods disclosed in Japanese Patent Application, First Publication No. (A) Hei 8-188679 (WPI Acc No. 96-322827); Japanese Patent Application, First Publication No. (A) Hei 8-193148 (WPI Acc No. 96-322827); and Japanese Patent Application, First Publication No. (A) Hei 10-87910 (WPI Acc No. 98-267240). This invention is one of the applied patents of these inventions.
In other words, it is the objective of the present invention to provide a laminate which can be produced without using a dry laminate adhesive agent or an extrusion laminate AC agent, in a small number of steps, with good productivity, this laminate having excellent gas barrier properties and odor retention. It addition, it is also the objective of the present invention to provide a production method for this laminate, and to provide a paper container employing this laminate.
The present inventors discovered that a resin composition containing epoxy group (xe2x80x9cepoxy-group-containing resin compositionxe2x80x9d hereinafter), in which a specific compound bearing an epoxy group has been mixed with a polyolefin derived resin, can be employed to adhere layers where a dry laminate adhesive agent or an extrusion laminate AC agent had been required conventionally. The present inventors learned that by means of this approach, it was possible to obtain a high adhesive strength without using an adhesive agent, to eliminate the need for time to cure (aging time) the adhesive agent during the production process, to improve production efficiency, and to achieve a reduction in the number of production steps. In addition, in the present invention, contamination of the work environment by organic solvents when producing the laminate is not an issue. Further, since the laminate has a gas barrier layer, by employing this laminate in a paper container or the like, it is possible to provide a container in which the quality of the contents therein is not affected by oxidation.
In other words, the present invention is a laminate with gas barrier properties having at least a (I) paper layer; a (II) gas barrier layer; and an (III) epoxy-group-containing resin composition layer, which includes 100 parts by weight of polyolefin (a) and 0.01xcx9c5 parts by weight of epoxy compound (b) which has two or more epoxy groups in the molecule and has a molecular weight of 3000 or less; wherein the (II) gas barrier layer is in contact with the (III) epoxy-group-containing resin composition layer. The present invention further concerns the production method for the aforementioned laminate and a paper container employing this laminate. In addition, in an other embodiment of a laminate with gas barrier properties, the (III) epoxy-group-containing resin composition layer consists of an epoxy-group-containing resin composition layer containing an olefin polymer(c) having a functional group that can react with the epoxy group in the amount of 30 weight percent or less of the epoxy-group-containing resin composition.